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From Python to Rust, then Elixir

Recently I made an app to prepare for some kind of test I have to take. I would have use Anki for this purpose but I don’t want to spend too much time learing how to make an Anki package again. Instead, I spent much more time to create a new alternative, but at least I’ve learned some things new.

1. Porting Anki’s SuperMemo2 from Python to Rust

I’m not confident enough to launch my own spaced repetition algorithm yet so I decided to use SuperMemo2 in a version implemented by Anki desktop app. The Python code is on one scheduler file and one test file.

First thing first, we define an interface which is the core of the algorithm:

pub trait Sched {
    fn next_interval(&self, card: &Card, choice: Choice) -> i64;
    fn answer_card(&self, card: &mut Card, choice: Choice);

Card contains SuperMemo2’s parameters such as queue name, due time, lapses. Choice is an enum correspond to the answer we make, from Forgot to Easy.

next_interval shows estimated time to recall the card if we make a choice, answer_card is the real action when we make that choice.

The next following steps are to translate Python functions to Rust methods one by one. It is quite a smooth process, I feel like I am just adding type annotations to Python code instead of writing new Rust code. The result is a new Rust file, look quite the same as the old Python one.

2. Expose Rust API to Elixir Phoenix app

After having myself a spaced repetition package, I need to bring it to life. To tell the truth, I chose Rust to port to because I know I can import it from almost anywhere through Foreign Function Interface (FFI).

For no specific reasons, let’s make an Elixir app!

We use Rustler to build the bridge between Rust code and Erlang NIFs. The setup is quite simple. After some times, we get a new Elixir module with the APIs we need:

defmodule Memoet.SRS.Sm2 do
  @moduledoc """
  Sm2 API, calling Rust code
  When your NIF is loaded, it will override those functions below.

  use Rustler, otp_app: :memoet, crate: "sm2"
  alias Memoet.SRS.{Config, Scheduler, Card, Choices}

  @spec new(Config.t(), integer(), integer()) :: Scheduler.t()
  def new(_config, _day_cut_off, _day_today), do: error()

  @spec next_interval(Card.t(), Scheduler.t(), Choices.t()) :: integer()
  def next_interval(_card, _scheduler, _choice), do: error()

  @spec answer_card(Card.t(), Scheduler.t(), Choices.t()) :: Card.t()
  def answer_card(_card, _scheduler, _choice), do: error()

Another way would work is to compile Rust code to Wasm, and then import it from anywhere support Wasm runtime.

3. Summary

Rust is reducing the gap between languages. Maintaining the core logic in Rust if possible should be considered as a wise choice.

Written on May 15, 2021.